One of the earliest schemes used to locate a radio transmitter was through the use of a network of direction finding (DF) receivers which produce lines of bearing at each site. The geographical location of the transmitter is estimated through triangulation and this technique is in widespread use at the present time. During the 1960's, a new technology was developed to locate a radio transmitter at a single receiving site. The initial step in the single site radio location (SSL) process is to estimate the angle-of-arrival (AOA) of the radio signal at one receiving site, the AOA being characterized by azimuth and elevation. In the case of HF (2-30 MHz) communication, the next step is to estimate the height of the ionospheric reflecting layer, which is typically accomplished by a vertical or oblique incidence sounding. The distance to the transmitter is computed using the estimated elevation of the AOA and ionospheric height. Transmitter location is then determined from the estimated azimuth AOA and distance (or range).
As illustrated in FIG. 1, a major source of error in conventional SSL technology is the identification of the propagation path between the transmitting 110 and receiving sites 130. In the case of HF communication, one must consider the propogated wave reflection at layers of different density and different altitudes within the ionosphere 120, the most common being designated as E 121, F.sub.1 122 and F.sub.2 123. If it is assumed that the signal propagated along path 124 via the E 121 layer, when in fact propagation occurred along path 125, via the F.sub.1 122 layer, the error in range estimate could be 100% or greater.
One object of this invention is to provide a means for resolving the issue of mode identification through passive measurements on the received signal and to significantly reduce location errors which result from misidentification of the propagation mode. A propagation phenomenon which routinely occurs in HF communication is that of multipath propagation. In the case of multipath, the signal may propogate from the transmitter 110 to the receiver 130 via two or more signal paths simultaneously, say path 124 and path 125, or path 125 and path 126, or all three paths 124, 125 and 126. In the case of multipath propagation, the wavefield at the receiving site 130 will be a superposition of the component waves arriving via signal paths 124, 125 and 126, for example. At the receiving site 130, this is generally referred to as a multicomponent wavefield. Consideration of multipath propagation represents an important distinction between the invention described in this disclosure and previous inventions. In previous disclosures it was generally assumed that the signal propagated between the transmitter and receiver along a single path. In this disclosure it is assumed that the signal propagates between the transmitter and receiver along two or more signal paths so that the signal observed at one of the receiving antennas 131, 132 or 133, contains a component arriving via a shorter length path 124 plus one or more echoes arriving via a shorter length path 124 plus one or more echoes arriving via a longer length path 125 and/or 126. In this invention the presence of the echoes is exploited, while in previous disclosures the echoes are either ignored or assumed not to exist.
In contrast to the conventional SSL technique, multipath propagation through the ionosphere can be exploited to accomplish passive location without requiring ionospheric height data. The invention described herein is a means to accomplish passive SSL by sampling the received signal at two or more antennas and to estimate the intersensor and interpath delay times. Intersensor delay is defined to be the time difference of arrival (TDOA) for a signal to arrive at receiving antenna 131 and antenna 132, say, or the difference between antennas 131 and 133. Clearly, the intersensor TDOA will be dependent upon the AOA of the signal relative to the spatial positioning of the antenna pair. The interpath delay is the differential transient time for a signal to propagate between the transmitter 110 and receiver 130 along, say, paths 124 and 125. Interpath delay time will be dependent upon the differential path length of the two signal paths and the velocity of propagation. If the path length 124, 125 and 126 are long relative to the spacing between receiving antennas 131, 132 and 133, then the interpath delay time observed at each receiving antenna will be essentially the same value. Stated another way, the time between echoes due to multipath as observed at each receiving antenna will be the same.
The intersensor and interpath delays are used to compute the location of the transmitter. To illustrate the one aspect of the technique, assume that a signal is propagating from the transmitting site 110 to the receiving site 130 along paths 124 and 126, simultaneously. Let receiving antenna 131 be a time reference for arrival so that the intersensor delays at antennas 132 and 133 can be determined for the E 121 components and the F.sub.2 123 component. Thus for the E 121 wave, there is a intersensor delay between antennas 131 and 132, likewise an intersensor delay exists between antennas 131 and 133. Because a line drawn between antennas 131 and 132 is oriented differently from a line drawn between antennas 131 and 133, there are two independent measurements of intersensor delay for the E-layer 121 wave. The two intersensor delays can be expressed as two simultaneous equations involving two unknowns and solved for the azimuth/elevation AOA of the E layer wave. Similarly, for the F.sub.2 123 wave, intersensor delays between antennas 131 and 132 and antennas 131 and 133 can be used to solve for ezimuth/elevation AOA appropriate for signal path 126.
The interpath delay between signal paths 124 and 126 may be estimated using an arbitrary pair of receiving antennas, say 131 and 132, and may be accomplished concurrent with the estimation of intersensor delay times. To solve for the location of the transmitter 110, the data measured at the receiving site 130 are used in concert, namely, the azimuth/elevation AOA of the path 124, the azimuth/elevation AOA of path 126 and the differential path length between paths 124 and 126 (computed from the interpath delay time and an assumed velocity of propagation). It is assumed that the launch angle of each signal path at the transmitter site 110 is the complement of the AOA observed at the receiving site 130. Further, it is assumed that the propagation is one hop skywave (as opposed to two hop skywave with an intervening ground reflection). Moreover, it is assumed that the reflection at the path midpoint obeys Snell's Law. If the transmitter 110 is located in close proximity to the receiver 130 so that a fiat earth model may be used, the solution for the location of the transmitter may be obtained in closed form. If the transmitter 110 is located at considerable distance from the receiver 130 so that a spherical earth model must be employed, then the solution for the location of the transmitter 110 results in a transcendental equation usually be solved iteratively.
In the preferred embodiment which is described in detail below, an example will be given utilizing crossed loop antennas deployed at multiple locations in a common plane. The several antennas thus cooperate to provide AOA information which is processed with the cooperative receiver system to provide delineation between layer reflections in the ionosphere so that errors based on incorrect determination of the ionospheric reflection are avoided.